1. Field of the Invention
The present invention relates to a printer arranged to print on any printing media of different widths while feeding the printing medium with a platen roller so that one side edge of the printing medium is positioned near one end of a thermal head.
2. Description of Related Art
Some conventional printers are arranged to print on any roll sheet or paper having different widths while feeding the sheet or paper with a platen roller so that one side edge of the sheet or paper is positioned near one end of a thermal head.
For instance, there has been proposed a printer provided with a printing unit including a thermal head, a thermal ink sheet to be supplied through a supply shaft and a winding shaft which are placed on both sides of the thermal head, and a transfer roller pressed against the thermal head. In this printer, a copy paper is transported to a nip point formed between the thermal ink sheet and the transfer roller, the paper being guided as held so that one side edge of the paper is positioned near one end of the thermal head. Only a pressing force of the transfer roller exerted on the thermal head on a side to which the paper is guided and fed can be adjusted to increase or decrease (for example, Japanese patent application laid-open publication No. H1(1989)-170164).
With this structure, the pressing force on the guided and fed side is reduced if the paper is a thin sheet and increased if the paper is a thick, particularly, narrow sheet (e.g., a postcard, a business card, etc.). Accordingly, various types of paper different in width and thickness can uniformly be printed.
In the above conventional printer, the platen roller is moved up and down to press the thermal head. This may cause displacement between the thermal head and the platen roller and displacement of components for a driving system. Thus, the printing accuracy and the feeding accuracy are likely to deteriorate.
To avoid the above disadvantages, for example, a tape printer 101 as shown in FIGS. 17A and 17B has been proposed. In this printer, a platen roller 102 is rotatably supported on side walls 103 and 104 of a frame through bearings 105 and 106 respectively. A thermal head 109 mounted on a head support member 108 is pressed against a peripheral surface of the platen roller 103 by coil springs 110 and 111. The coil springs 110 and 111 are formed to have a substantially equal pressing force and placed to divide substantially equally each length from a center of the thermal head 109 to each end thereof in a width direction.
As shown in FIG. 17A, in the case where a wide roll sheet 112 having a substantially equal width to the length of the thermal head 109 is fed while a side edge (a right edge in FIG. 17A) of the roll sheet 112 is positioned in a reference point 113 near one end (a right end in FIG. 17A) of the thermal head 109 in its longitudinal direction, the thermal head 109 can be brought into contact under substantially uniform pressure with the entire roll sheet 112.
As shown in FIG. 17B, however, in the case where a narrow roll sheet 112 having a width about half the length of the thermal head 109, the head support member 118 would be inclined when a side edge (a right edge in FIG. 17B) of the roll sheet 112 is positioned in the reference point 113. The thermal head 109 cannot be pressed under uniform pressure against the entire roll sheet 112. This would cause a large reduction in print quality of the roll sheet 112 on the reference point 113 side.